We know that the SCR has two stable states as forward blocking and forward conduction state. Switching the SCR from forward blocking state (OFF- state) to forward conduction state (ON- state) is known as turning ON process of SCR . It is also called as triggering.
The criteria for triggering the SCR depends on the several variables like supply voltage, gate current, temperature, etc. There are various methods to trigger the SCR so that it comes into the ON state. Let us discuss these methods in brief.
With a voltage applied to the SCR, if the anode is made positive with respect to the cathode, the SCR becomes forward biased. Thus, the SCR comes into the forward blocking state. The SCR can be made to conduct or switching into conduction mode is performed by any one of the following methods.
- Forward voltage triggering
- Temperature triggering
- dv/dt triggering
- Light triggering
- Gate triggering
By increasing the forward anode to cathode voltage, the depletion layer width is also increasing at junction J2. This also causes to increase the minority charge carriers accelerating voltage at junction J2. This further leads to an avalanche breakdown of the junction J2 at a forward breakover voltage VBO.
At this stage SCR turns into conduction mode and hence a large current flow through it with a low voltage drop across it. During the turn ON state the forward voltage drop across the SCR is in the range of 1 to 1.5 volts and this may be increased with the load current.
In practice this method is not employed because it needs a very large anode to cathode voltage. And also once the voltage is more than the VBO, it generates very high currents which may cause damage to the SCR. Therefore, most of the cases this type of triggering is avoided.
The reverse leakage current depends on the temperature. If the temperature is increased to a certain value, the number of hole-pairs also increases. This causes to increase the leakage current and further it increases the current gains of the SCR. This starts the regenerative action inside the SCR since the (α1 + α2) value approaches to unity (as the current gains increases).
By increasing the temperature at junction J2 causes the breakdown of the junction and hence it conducts. This triggering occur in some circumstances particularly when it the device temperature is more (also called false triggering). This type of triggering is practically not employed because it causes the thermal runaway and hence the device or SCR may be damaged.
In forward blocking state junctions J1 and J3 are forward biased and J2 is reverse biased. So the junction J2 behaves as a capacitor (of two conducting plates J1 and J3 with a dielectric J2) due to the space charges in the depletion region. The charging current of the capacitor is given as
I = C dv/ dt
where dv/dt is the rate of change of applied voltage and C is the junction capacitance.
From the above equation, if the rate of change of the applied voltage is large that leads to increase the charging current which is enough to increase the value of alpha. So the SCR becomes turned ON without a gate signal.
However, this method is also practically avoided because it is a false turn ON process and also this can produce very high voltage spikes across the SCR so there will be considerable damage to it.
An SCR turned ON by light radiation is also called as Light Activated SCR (LASCR). This type of triggering is employed for phase controlled converters in HVDC transmission systems. In this method, light rays with appropriate wavelength and intensity are allowed to strike the junction J2.
These types of SCRs are consisting a niche in the inner p-layer. Therefore, when the light struck on this niche, electron-hole pairs are generated at the junction J2 which provides additional charge carriers at the junction leads to turn ON the SCR.
This is most common and efficient method to turn ON the SCR. When the SCR is forward biased, a sufficient voltage at the gate terminal injects some electrons into the junction J2. This result to increase reverse leakage current and hence the breakdown of junction J2 even at the voltage lower than the VBO.
Depends on the size of the SCR the gate current varies from a few milli-amps to 200 milli amps or more. If the gate current applied is more, then more electrons are injected into the junction J2 and results to come into the conduction state at much lower applied voltage.
In gate triggering method, a positive voltage applied between the gate and the cathode terminals. We can use three types of gate signals to turn On the SCR. Those are DC signal, AC signal and pulse signal.
In this triggering, a sufficient DC voltage is applied between the gate and cathode terminals in such a way that the gate is made positive with respect to the cathode. The gate current drives the SCR into conduction mode. In this, a continuous gate signal is applied at the gate and hence causes the internal power dissipation (or more power loss).
This is the most commonly used method for AC applications where the SCR is employed for such applications as a switching device. With the proper isolation between the power and control circuit, the SCR is triggered by the phase-shift AC voltage derived from the main supply. The firing angle is controlled by changing the phase angle of the gate signal.
However, only one half of the cycle is available for the gate drive to control the firing angle and next half of the cycle a reverse voltage is applied between the gate and cathode. This is one of the limitation of AC triggering and also separate step down or pulse transformer is needed to supply the voltage to gate drive from the main supply.
The most popular method of triggering the SCR is the pulse triggering. In this method, gate is supplied with single pulse or a train of pulses.
The main advantage of this method is that gate drive is discontinuous or doesn’t need continuous pulses to turn the SCR and hence gate losses are reduced in greater amount by applying single or periodically appearing pulses. For isolating the gate drive from the main supply, a pulse transformer is used.
The dynamic processes of the SCR are turn ON and turn OFF processes in which both voltage and currents of an SCR vary with time. The transition from one state to another takes finite time, but doesn’t take place instantaneously.
The static or VI characteristics of the SCR give no indication about the speed at which the SCR switched into forward conduction mode from forward blocking mode. Hence the dynamic characteristics are sometimes more important which gives the switching characteristics of the SCR.
There will be a finite transition time that SCR takes to reach the forward conduction mode from blocking mode, which is termed as turn ON time of SCR. The turn ON time of the SCR Ton can be subdivided into three distinct intervals namely delay time td, rise time tr, and spread time ts.
Delay Time (td)
The delay time is measured from the instant at which the gate current reaches 90 percent of its final value to the instant at which anode current reaches 10 percent of its final value. It can also define as the time between which anode voltage falls from initial anode voltage value Va to 0.9 VA .
Consider the below figure and observe that, until the time td, the SCR is in forward blocking mode so the anode current is the small leakage current. When the gate signal is applied (at 90 percent of Ig) then the gate current is reached to 0.1 Ia and also correspondingly anode to cathode voltage falls to 0.9 VA.
With the gate signal applied, there will be non-uniform distribution of current over the cathode surface so the current density is much higher at gate terminal. And it rapidly decreases as the distance from gate increases. Hence, the delay time td is the time during which anode current flows in a narrow region at which current density(gate current) is highest.
Rise Time (tr)
This is the time taken by the anode current to rise from 10 percent to 90 percent of its final value. Also called as the time required for the forward blocking voltage to fall from 0.9Va to 0.1 VA. This rise time is inversely proportional to the gate current and its rate of building up.
Therefore, if high and steep current pulses are applied at the gate reduces the rise time tr. Also, if the load is inductive this rise time will be higher and for resistive and capacitive loads it is low.
During this time, turn ON losses in the SCR are high due to large anode current and high anode voltage occurs simultaneously. This can result in the formation of local hot spots and hence the SCR may be damaged.
Spread Time (ts)
This is the time taken by the anode current to rise from 0.9Ia to Ia. Also the time required for the forward blocking voltage to fall from o.1Va to its ON-state voltage drop which is the range of 1 to 1.5 volts. During this time anode current spread over the entire conducting region of an SCR from a narrow
conducting region. After the spreading time, a full anode current flows through the device with small ON-state voltage drop.
Therefore, the total turn ON time,
Ton = tr + td + ts
The typical value of the turn ON time is in the order of 1 to 4 micro seconds depends on the gate signal wave shapes and anode circuit parameters . To reduce the turn ON time of the SCR, the amplitude of the gate pulse should be in the order of 3 to 5 times the minimum gate current of the SCR.
As we have seen in above that out of various triggering methods to turn the SCR, gate triggering is the most efficient and reliable method. Most of the control applications use this type of triggering because the desired instant of SCR turning is possible with gate triggering method. Let us look on various firing circuits of SCR.
Resistance Firing Circuit
- The circuit below shows the resistance triggering of SCR where it is employed to drive the load from the input AC supply. Resistance and diode combination circuit acts as a gate control circuitry to switch the SCR in the desired condition.
- As the positive voltage applied, the SCR is forward biased and doesn’t conduct until its gate current is more than minimum gate current of the SCR.
- When the gate current is applied by varying the resistance R2 such that the gate current should be more than the minimum value of gate current, the SCR is turned ON. And hence the load current starts flowing through the SCR.
- The SCR remains ON until the anode current is equal to the holding current of the SCR. And it will switch OFF when the voltage applied is zero. So the load current is zero as the SCR acts as open switch.
- The diode protects the gate drive circuit from reverse gate voltage during the negative half cycle of the input. And Resistance R1 limits the current flowing through the gate terminal and its value is such that the gate current should not exceed the maximum gate current.
- It is the simplest and economical type of triggering but limited for few applications due to its disadvantages.
- In this, the triggering angle is limited to 90 degrees only. Because the applied voltage is maximum at 90 degrees so the gate current has to reach minimum gate current value somewhere between zero to 90 degrees.
Resistance – Capacitacne (RC) Firing Circuit
- The limitation of resistance firing circuit can be overcome by the RC triggering circuit which provides the firing angle control from 0 to 180 degrees. By changing the phase and amplitude of the gate current, a large variation of firing angle is obtained using this circuit.
- Below figure shows the RC triggering circuit consisting of two diodes with an RC network connected to turn the SCR.
- By varying the variable resistance, triggering or firing angle is controlled in a full positive half cycle of the input signal.
- During the negative half cycle of the input signal, capacitor charges with lower plate positive through diode D2 up to the maximum supply voltage Vmax. This voltage remains at -Vmax across the capacitor till supply voltage attains zero crossing.
- During the positive half cycle of the input, the SCR becomes forward biased and the capacitor starts charging through variable resistance to the triggering voltage value of the SCR.
- When the capacitor charging voltage is equal to the gate trigger voltage, SCR is turned ON and the capacitor holds a small voltage. Therefore the capacitor voltage is helpful for triggering the SCR even after 90 degrees of the input waveform.
- In this, diode D1 prevents the negative voltage between the gate and cathode during the negative half cycle of the input through diode D2.
- It is the most common method of triggering the SCR because the prolonged pulses at the gate using R and RC triggering methods cause more power dissipation at the gate so by using UJT (Uni Junction Transistor) as triggering device the power loss is limited as it produce a train of pulses.
- The RC network is connected to the emitter terminal of the UJT which forms the timing circuit. The capacitor is fixed while the resistance is variable and hence the charging rate of the capacitor depends on the variable resistance means that the controlling of the RC time constant.
- When the voltage is applied, the capacitor starts charging through the variable resistance. By varying the resistance value voltage across the capacitor get varied. Once the capacitor voltage is equal to the peak value of the UJT, it starts conducting and hence produce a pulse output till the voltage across the capacitor equal to the valley voltage Vv of the UJT. This process repeats and produces a train of pulses at base terminal 1.
- The pulse output at the base terminal 1 is used to turn ON the SCR at predetermined time intervals.